Phasors Handling the trigonometric functions can be tedious: This is just a simple phase shift, say, = t, = 0. We have Euler's identity: that relates trigonometric functions to complex exponentials, the math of which is a bit simpler: e j ( ) e j e j At least, phase shifting is much easier! So, we use ej to represent cos for mathematical convenience. But, why can we do this? What’s behind this? Based on this, we have (say, a voltage) This rotation part is always there. So leave it out. (or "complex amplitude) leave out. In both cases, the time dependence is left out, since we are dealing with a single frequency.